config: catch and report unexpected arguments

after handling arguments, catch and report any remaining "non-option"
arguments.

for instance users could make the basic mistake of using
`lb config --bootloaders syslinux grub-efi`, i.e. failing to quote the
multiple bootloaders (i actually encountered a user doing this who swore
that "it just worked"). catching and reporting such mistakes could be
valuable to users.

previous behaviour:
```
$ lb config --bootloaders syslinux grub-efi
P: Updating config tree for a debian/buster/amd64 system
P: Symlinking hooks...
```

new behaviour:
```
$ lb config --bootloaders syslinux grub-efi
[2020-05-05 18:56:07] lb config --bootloaders syslinux grub-efi
E: Unexpected argument found: grub-efi
```

Gbp-Dch: Short

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